Pressure regulating method

ABSTRACT

The pressure of hydraulic fluid which is supplied to the continuously variable transmission in the power train of a motor vehicle is regulated by an apparatus wherein an adjustable pilot valve can receive fluid from a pilot conduit leading to a pressure reducing valve. The pilot conduit normally receives fluid from a further conduit, wherein the pressure of fluid is at least substantially constant, by way of a flow restrictor. A branch conduit serves to evacuate fluid from the pilot conduit into the pilot valve when the latter is at least partially open. The fluid-discharging end of the flow restrictor confronts the inlet of the branch conduit, and such inlet is narrowed to reduce the likelihood of turbulence developing in the fluid stream flowing from the pilot conduit, across the further conduit and into the inlet of the pilot valve.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to improvements in pressureregulating methods and apparatus.

[0002] It is often necessary to design a hydraulic control system insuch a way that a regulating or control valve (e.g., a pressure reducingvalve) is installed downstream of a pilot valve or servo valve(hereinafter called pilot valve). The latter serves to select fluidpressure in a (pilot) conduit which is connected with an inlet of theregulating valve. An outlet of the regulating valve serves to conveyfluid to one or more consumers, e.g., to a so-called continuouslyvariable transmission (CVT). Reference may be had, for example, tocommonly owned U.S. Pat. No. 5,667,448 (granted Sep. 16, 1997 to OswaldFriedmann for “POWER TRAIN”) and to commonly owned U.S. Pat. No.5,674,155 (granted Oct. 7, 1997 to Dieter Otto et al. for “METHOD OF ANDAPPARATUS FOR TRANSMITTING TORQUE IN THE POWER TRAINS OF MOTORVEHICLES”). The disclosures of the just enumerated US patents as well asof all other patents and patent applications (including commonly ownedpending German priority patent application Ser. No. 198 29 148.5 filedJun. 30, 1998) are incorporated herein by reference.

[0003] A conventional pilot valve-regulating valve combination canemploy a pilot valve having an inlet connected to a source ofconstant-pressure (or substantially constant-pressure) fluid by aconstant-pressure conduit which contains a flow restrictor) e.g., anadjustable diaphragm, and can communicate with a pilot conduit servingto convey fluid to the regulating valve. An adjustable valving elementof the pilot valve can divert varying quantities of fluid from the pilotconduit (downstream of the flow restrictor) to a sump or to anotherdestination so that the pilot valve can select the pressure of fluidentering the body of the regulating valve.

[0004] A drawback of heretofore known pressure regulating methods andapparatus of the just outlined character is that (for example, due tooften unavoidable leakage of fluid from the pilot valve and/or fromother parts of the apparatus), the pressure in the pilot conduit(between the pilot valve and the regulating valve) cannot match thepressure of fluid in the constant-pressure conduit. Moreover, even ifthe valving element of the pilot valve is moved to a fully open position(in which the valving element does not interfere with the flow of fluidfrom the pilot conduit, through the pilot valve, and on to to the sump),the pilot pressure of fluid (in the pilot conduit) still exceeds zeropressure. Such residual pressure in the pilot conduit is attributable tolosses developing in the region of the flow restrictor, e.g., to lossesin the path defined by the constant-pressure conduit, by the flowrestrictor, by a branch conduit connecting the constant-pressure conduit(downstream of the flow restrictor) to the inlet of the pilot valve, andby the conduit connecting the outlet of the pilot valve with the sump orwith another receptacle for hydraulic fluid.

[0005] A further drawback of the aforedescribed conventional pressureregulating apparatus is that, due to turbulence which develops in thefluid path in the region of the flow restrictor, at least a highpercentage of kinetic energy of the fluid (such as oil) is ultimatelyconverted into heat.

OBJECTS OF THE INVENTION

[0006] An object of the invention is to provide a novel and improvedmethod of regulating the pressure of hydraulic fluid which is utilizedin cylinder-and-piston units and/or other constituents of transmissionsand/or other fluid-operated components, e.g., in the power trains ofmotor vehicles.

[0007] Another object of the invention is to provide a method which canbe practiced without the generation of appreciable, noticeable or evenany quantities of heat.

[0008] A further object of the invention is to provide a method whichcan be practiced by resorting to a relatively simple, compact andinexpensive apparatus.

[0009] An additional object of the invention is to provide a methodwhich renders it possible to reduce the pilot pressure between the pilotvalve and the regulating valve of the appparatus to a threshold valuebelow that achievable by resorting to heretofore known methods.

[0010] Still another object of the invention is to provide an apparatuswhich can be utilized for the practice of the above outlined method andis designed in such a way that the aforedescribed drawbacks ofconventional apparatus can be avoided in a simple, effective andinexpensive manner.

[0011] A further object of the invention is to provide a novel andimproved pilot valve-regulating valve combination for use in the aboveoutlined apparatus.

[0012] Another object of the invention is to provide the improvedapparatus with a novel combination of a flow restrictor and conduits forconveying hydraulic fluid from a source of fluid at an at leastsubstantially constant pressure (e.g., from a valve) to the pilot valveand from the pilot valve to the regulating valve.

[0013] An additional object of the invention is to provide an apparatuswhich can be utilized in the power trains of motor vehicles as asuperior substitute for the aforedescribed and other conventionalpressure regulating apparatus.

[0014] Still another object of the invention is to provide afluid-consuming or fluid-operated unit, such as a continuously operabletransmission, which embodies an apparatus of the above outlinedcharacter.

[0015] A further object of the invention is to provide an apparatuswhich exhibits the advantages but does not embody the drawbacks ofstandard fluid pressure regulating apparatus utilizing combinations ofpilot valves and regulating valves in the path of hydraulic fluid flowfrom a source of constant-pressure fluid to a consumer requiring fluidat two or more different pressures.

[0016] Another object of the invention is to provide a motor vehicle,such as a passenger car, having a power train embodying a unit whichconsumes or is operated by a hydraulic fluid and which receiveshydraulic fluid from the above outlined apparatus in accordance with amethod of the present invention.

SUMMARY OF THE INVENTION

[0017] One feature of the instant invention resides in the provision ofa method of controlling the pressure of a hydraulic fluid (e.g., a fluidwhich includes or constitutes a lubricant such as oil) in a pilotconduit which receives fluid at an at least substantially constantpressure from a constant-pressure conduit by way of a flow restrictorand a pilot valve which is adjustable to permit outflow of fluid fromthe pilot conduit through a branch conduit at a variable rate and tothus lower the static pressure of fluid in the pilot conduit in responseto increasing rate of fluid outflow from the pilot conduit. The improvedmethod comprises the steps of (a) establishing for the flow of fluidfrom the pilot conduit to the pilot valve a path extending from thepilot conduit first through the flow restrictor and thereupon throughthe branch conduit into and through the pilot valve, and (b) varying thespeed of the fluid flowing through the flow restrictor to thus influencethe static pressure of fluid in the pilot conduit.

[0018] The speed varying step can include adjusting the pilot valve toselect the rate of fluid flow from the branch conduit into the pilotvalve. The fluid which is permitted to flow from the pilot conduit,through the branch conduit and through the pilot valve can be confinedin (i.e., collected by) a suitable receptacle, e.g., in a sump.

[0019] Another feature of the invention resides in the provision of anapparatus for controlling the pressure of a hydraulic fluid, e.g., atransmission fluid, a lubricant or the like. The apparatus comprises apilot valve having an inlet, an outlet and means for varying the rate offluid flow from the inlet to the outlet, a source of constant-pressurehydraulic fluid including a first conduit having an outlet, a branchconduit connecting the outlet of the first conduit with the inlet of thepilot valve, a pilot conduit which is arranged to receive fluid from thebranch conduit and wherein the static pressure of fluid decreases inresponse to increasing rate of fluid outflow from the pilot conduit, anda flow restrictor which establishes an evacuating path for the outflowof fluid from the pilot conduit into the branch conduit.

[0020] The flow restrictor is installed between the first conduit andthe pilot conduit. The latter establishes a second path for the flow offluid to at least one of the first conduit and the branch conduit; theevacuating path can be oriented in such a way that it crosses the secondpath.

[0021] In accordance with a presently preferred embodiment, the firstconduit includes a portion which constitutes a portion of the pilotconduit (for example, such portions of the first and pilot conduits canbe disposed back-to-back), and the flow restrictor is provided in suchportion of the first conduit. The inlet of the branch conduit in suchapparatus is positioned to receive fluid flowing along the evacuatingpath from the pilot conduit, through the flow restrictor and across thefirst conduit; the outlet of such branch conduit is connected with theinlet of the pilot vave. The dimensions of the just described branchconduit can be selected in such a way that its inlet has a firstcross-sectional area and that the branch conduit further includes aportion adjacent the inlet of the branch conduit (e.g., extending allthe way from the inlet of the branch conduit and to the inlet of thepilot valve) and having a second cross-sectional area greater than thefirst cross-sectional area.

[0022] The just described inlet and portion of the branch conduitconstitute a means for opposing the development of turbulence (such asvortices) in the evacuating path.

[0023] The means for varying the rate of fluid flow from the inlet tothe outlet of the pilot valve can include a valving element (e.g., apiston which is reciprocable in the housing or body of the pilot valve)and means for moving the valving element relative to the body of thepilot valve. The moving means can comprise at least one magnet, e.g., anelectromagnet.

[0024] The orientation of the first conduit and the pilot conduit can besuch that these conduits include portions which are at leastsubstantially normal to the branch conduit. The latter can have at leastone bend between the flow restrictor and the inlet of the pilot valve.Alternatively, the branch conduit can be a straight conduit having aninlet communicating with the pilot conduit and an outlet communicatingwith the inlet of the pilot valve.

[0025] The improved apparatus can further comprise a regulating valvehaving an inlet arranged to receive fluid from the pilot conduit and anoutlet, a consumer of fluid, and a control conduit connecting the outletof the regulating valve with at least one inlet of the consumer. Forexample, the consumer can comprise a transmission in the power train ofa motor vehicle, e.g., a so-called continuously variable transmission(CVT). A typical example of a continuously variable transmission is atransmission employing two parallel shafts each of which carries anadjustable pulley, and an endless belt or chain which is trained overthe pulleys. The fluid which is supplied by the regulating valve by wayof the control conduit can be utilized to vary the ratio of thetransmission and/or to select the extent of frictional engagementbetween the flanges of the adjustable pulleys and endless chain or belt.

[0026] The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theiproved apparatus itself, however, both as to its construction and itsmode of operation, together with numerous additional features andattributes thereof, will be best understood upon perusal of thefollowing detailed description of certain presently preferred specificembodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a fragmentary schematic partly elevational and partlysectional view of a conventional fluid pressure regulating apparatus;

[0028]FIG. 2 is a diagram wherein the curve represents the relationshipbetween the changes of fluid pressure in the pilot conduit and thechanges in the setting of the pilot valve in the apparatus of FIG. 1;

[0029]FIG. 3 is a fragmentary schematic partly elevational and partlysectional view of second conventional apparatus;

[0030]FIG. 4 is a view similar to that of FIG. 3 but showing certainconstituents of an apparatus which embodies one form of the invention;

[0031]FIG. 5 is a diagram wherein the curve denotes the relationshipbetween the changes of pressure in the pilot conduit and the changes inthe setting of the pilot valve in the apparatus of FIG. 4; and

[0032]FIG. 6 is a fragmentary schematic partly elevational and partlysectional view of a novel apparatus which constitutes a modification ofthe apparatus shown in FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0033]FIG. 1 shows certain component parts of a conventional apparatuswhich includes a pump 6 serving to draw hydraulic fluid (such as oil andhereinafter referred to as fluid) from a source 7, e.g., a sump. Theoutlet of the pump 6 delivers a stream of fluid into a main conduit 8 ata pressure which can exceed the pressure required in a control conduit28 serving to deliver at least one stream of pressurized fluid to atleast one inlet of a consumer 29, e.g., a continuously variabletransmission (CVT) of the type disclosed in the commonly owned U.S. Pat.No. 5,667,448 or 5,674,155.

[0034] The main conduit 8 supplies pressurized fluid to an inlet of aconstant-pressure valve 10 (this valve can be replaced with a pressureregulating valve, not shown) having an outlet connected with the inletor intake end of a (first) constant-pressure conduit 12. The valve 10supplies fluid at an at least substantially constant pressure. Thedischarge end or outlet of the constant-pressure conduit 12 deliversfluid to the inlet of a pilot conduit 20 by way of a flow restrictor ofthrottle 14 having a passage with a fixed or variable effectivecross-sectional area for the flow of fluid from the constant-pressurevalve 10 to the pilot conduit 20.

[0035] The outlet of the pilot conduit 20 (i.e., the outlet of theconstant-pressure conduit 12 downstream of the flow restrictor 14) isfurther connected with an inlet of a pilot valve 16 by a branch conduit22. The valve 16 has an outlet 24 which can discharge fluid into thesump 7 when a reciprocable valving element (such as a piston) 18 in thehousing or body 19 of the valve 16 permits at least some fluid to flowalong a path extending from the pilot conduit 20, through the branchconduit 22, between the inlet of the housing 19 and the outlet 24, andfrom the outlet 24 into the sump 7.

[0036] The valving element 18 is movable by a magnet, preferably anelectromagnet having at least one coil 30 arranged to receive electriccurrent from a suitable source 32. The sump 7 (or at least that portionof the sump which can receive fluid from the outlet 24) can be replacedwith a tank or any other suitable receptacle for hydraulic fluid whichflows along the aforementioned (evacuating) path from the pilot conduit20 to and beyond the outlet 24 when the pilot valve 16 is at leastpartially open. At such time, the valving element or piston 18 is spacedapart from a seat of the housing 19; the seat is adjacent the respectiveend of the branch conduit 22, i.e., it is located at the left-hand endof the cylinder chamber for the reciprocable valving element 18 in thehousing 19.

[0037] When the pump 6 is driven and the pilot valve 16 is at leastsubstantially sealed, the conduit 12 delivers a stream of fluid to thepilot conduit 20 via flow restrictor 14. The outlet of the conduit 20delivers fluid to one inlet of a pressure reducing valve 26 (e.g., aslide valve) having an outlet connected with the consumer 29 by way ofthe aforementioned control conduit 28. The valve 26 regulates thepressure of fluid in the conduit 28 and in the consumer 29.

[0038] The exact construction of the valves 10, 16 and 26 is known and,therefore, need not be described here. The pilot valve 16 is normallyclosed, i.e., the valving element 16 dwells in the left-hand endposition so that the branch conduit 22 is at least substantially sealedfrom the outlet 24 in the housing 19 of the valve 16. The valvingelement 18 is held in the left-hand end position when the intensity ofthe magnetic field established by the coil or coils 30 assumes a maximumvalue.

[0039]FIG. 3 shows a portion of another conventional fluid pressureregulating apparatus. All such parts which are identical with or clearlyanalogous to those shown in FIG. 1 are denoted by similar characters.The only difference is that straight portions of the conduits 12, 20 areadjacent each other and that the flow restrictor 14 is arranged toconvey fluid between such straight portions.

[0040] The mode of operation of the apparatus shown in FIGS. 1 and 3will be explained with reference to the coordinate system of FIG. 2wherein the fluid pressure (in bar) in the pilot conduit 20 is measuredalong the ordinate, and the current (in Amperes) supplied to the coil orcoils 30 is measured along the abscissa.

[0041] It is assumed that the fluid pressure in the first conduit 12 isat least substantially constant and amounts to between about 5 and 5.3bar. The flow restrictor 14 can constitute an adjustable diaphragm andis assumed to be set for the establishment of a circular orsubstantially circular opening with a diameter of approximately 0.8 mm.

[0042] When the intensity of the magnetic field established by the coilor coils 30 reaches a maximum value (e.g., when the current furnished tothe terminal(s) 32 has a strength in the range of one Ampere, thevalving element 18 engages its seat and thus seals or substantiallyseals the branch conduit 22 from the outlet 24 of the housing 19.Consequently, the pressure of fluid in the conduits 20 and 22 ismaintained at a maximum value, e.g., at slightly less than 5 bar. Thepressure of fluid in the conduits 20, 22 cannot rise to the pressure offluid in the first (constant-pressure) conduit 12 due to leakage of thepilot valve 16 and/or due to other leak or leaks in the apparatusembodying the structure shown in FIG. 1 or FIG. 3.

[0043] If the strength of the current flowing through the coil or coils30 is reduced, the valving element 18 proceeds to move away from theseat in the housing 19. This establishes a flow of fluid from the pilotconduit 20, through the branch conduit 22, from the inlet to the outlet24 of the housing 19, and into the sump 7. When the pilot valve 16 isfully open, the residual pressure of fluid in the pilot conduit 20amounts to between about 0.2 and 0.4 bar. The existence of such residualpressure is attributable to friction between the fluid in the path fromthe pilot conduit 20 into the sump 7 (via pilot valve 16), and thesurrounding surfaces.

[0044] Furthermore, and as shown in FIG. 3 by arrows 15, kinetic energyof the fluid flowing through the flow restrictor 14 causes thegeneration of turbulence (such as eddies or vortices immediatelydownstream of the flow restrictor) which, in turn, entails a heating ofthe conduits 12, 20 and a heating of the conveyed fluid.

[0045] The characteristic curve 40 in the coordinate system of FIG. 2indicates that, when the valving element 18 is remote from its seat inthe housing 19, the relationship between the amperage of the currentsupplied by the coil or coils 30 and the pressure in the pilot valve 16deviates from a linear relationship.

[0046]FIG. 4 shows a portion of an apparatus which embodies one form ofthe present invention. The reference characters utilized in FIG. 4 arelargely identical with those shown in FIG. 1 or 3 except for the numeral34 which denotes a portion or wall of the conduit 20 of one piece withand thus constituting a portion of the conduit 20. Furthermore, the flowrestrictor or nozzle 14 is provided in the common portion 34 of theconduits 12, 20 in such a way that it can discharge fluid from theconduit 12, across the conduit 20 and directly into the inlet of thebranch conduit 22. The latter has a bend 22 a between its inlet (at 14)and its outlet (i.e., the inlet of the pilot valve 16). Thus, when thevalve 16 is open to an extent determined by the intensity of magneticfield being then established by the coil or coils 30, the fluid can flowfrom the constant-pressure conduit 12, across the pilot conduit 20 (viaflow restrictor 14) and directly into the inlet of the branch conduit 22to thereupon flow to the sump 7 along the path established by thehousing 19 of the pilot valve 16.

[0047] The speed of the fluid flowing from the conduit 12 into theconduit 22 across the conduit 20 is very high (because the orificedefined by the flow restrictor 14 is preferably small). Due to suchelevated speed of the fluid flowing into the branch conduit 22, thestatic pressure in the conduit 20 is reduced in accordance with theBernoulli theorem; this causes the flow restrictor 14 to act as a jetpump and to draw liquid from the conduit 20 into the branch conduit 22.

[0048]FIG. 5 shows a coordinate system corresponding to that shown inFIG. 2, except that the fluid pressure (measured in bar) in the conduit20 of FIG. 4 is related to the amperage of the current supplied to thecoil(s) 30 shown in FIG. 4. When the pilot valve 16 of FIG. 4 is atleast nearly fully open, the pressure in the pilot conduit 20 dropsbelow the pressure indicated in FIG. 2. Thus, and a shown in FIG. 5, thepressure can drop below the zero value. This ensures that therelationship between the pilot pressure in the conduit 20 and theamperage of the current being supplied to the coil(s) 30 remains lineareven when the fluid pressure in the pilot conduit 20 is very low, i.e.,when the valving element 18 is remote from its seat in the valve housing19. This amounts to a widening of the operating range of the improvedapparatus beyond the operating range of conventional apparatus. When thepressure of fluid in the pilot conduit 20 is relatively high, thedifference between the corresponding portions of the curves 40 and 40Arespectively shown in FIGS. 2 and 5 is or can be negligible. The reasonis that when the fluid pressure in the conduit 20 is relatively high(because the valve 16 is fully or nearly fully closed), the speed offluid flow from the conduit 20 via conduit 22 and valve 16 (in theapparatus of FIG. 4) is low or negligible.

[0049] An advantage of the improved method and apparatus is that theyrender it possible to widen or broaden the operating range within whichone can control, by means of the pilot pressure, the operations of othervalves such as slide valves or the pressure reducing valve and/or othercomponents of the apparatus and the power train. Moreover, one canestablish a more predictable linear relationship between the pressure inthe pilot conduit 20 and the (magnetic) force with which the valvingelement 18 is urged to its operative (sealing) position of engagementwith the seat in the housing 19 to thus prevent or reduce the rate offluid flow from the pilot conduit 20, through the branch conduit 22 andout of the housing 19 via outlet 24.

[0050] It is clear that the aforedescribed advantages of the improvedmethod and apparatus become even more pronounced if the velocity of thefluid stream flowing from the pilot conduit 20, through the branchconduit 22 and through the housing 19 of the pilot valve 16 increases.

[0051]FIG. 6 illustrates a portion of an apparatus which constitutes amodification of the novel apparatus shown in FIG. 4. One differencebetween the apparatus of FIGS. 4 and 6 is that the branch conduit 22 ofFIG. 6 includes an end portion 36 merging into the adjacent straightportion of the pilot conduit 20 and a straight portion 22A which extendsto the inlet of the body or housing 19 of the pilot valve 16. Thecross-sectional area of the passage defined by the inlet 36 is smallerthan that of the passage defined by the portion 22A. The straight branchconduit 22 of FIG. 6 renders it possible to dispense with the bend ofthe type shown at 22 a in FIG. 4. A bend (such as 22 a) might benecessary under certain circumstances (e.g., to take advantage of thespace which is avaiable for the apparatus); however, such bend or bendsnecessarily affect the velocity of the fluid flowing from the pilotconduit 20 toward and into the valve 16.

[0052] An advantage of the smaller-diameter inlet 36 of the branchconduit 22 of FIG. 6 is that it enhances the velocity of the fluidstream flowing through the conduit 22 and into the body 19 of the pilotvalve 16. The internal surface of the inlet 36 is preferably rounded andsmooth. Moreover, the inlet 36 enables the branch conduit 22 of FIG. 6to act as a highly effective jet pump, i.e., a pump generating a suctionmuch more pronounced than that achievable with the branch conduit 22 andflow restrictor 14 shown in FIG. 1 or 3.

[0053] The flow restrictor 14 of FIG. 6 is provided in that portion ofthe constant-pressure conduit 12 which is common to the pilot conduit20, the same as in FIG. 4.

[0054] It has been found that the operation of the improved apparatus isparticularly satisfactory (especially as concerns savings in enery) ifthe pressure in the pilot conduit 20 is zero bar when the currentstrength in the coil or coils 30 is at least very close to 0 mA.

[0055] It is also desirable to design the improved apparatus in such away that the pilot pressure in the conduit 20 is zero bar when thestrength of the current flowing in the coil or coils 30 is slightlyabove 0 mA. This is of particular advantage in the event of variationsof temperature and/or viscosity of the conveyed fluid. Under suchcircumstances, the 0-bar operating point can be caused to conform (i.e.,it can be adapted) to the operating conditions by varying the strengthof the current in the windings of the coil or coils 30.

[0056] To summarize, one of the important advantages of the improvedmethod and apparatus is that the fluid pressure in the pilot conduit 20can be lowered below that in the pilot conduit of a conventionalapparatus when the pilot valve 16 offers little or no resistance to theflow of fluid from the conduit 20, via conduit 22, through the valve 16and into the sump 7. This is accomplished in that the kinetic energy ofthe stream of fluid flowing from the conduit 12, through the flowrestrictor 14 and branch conduit 22 into and from the valve 16 isutilized to further reduce the fluid pressure in the conduit 20.

[0057] Without further analysis, the foregoing will so fully reveal thegist of the present invention that others can, by applying currentknowledge, readily adapt it for various applications without omittingfeatures that, from the standpoint of prior art, fairly constituteessential characteristics of the generic and specific aspects of theabove outlined contribution to methods of and apparatus for regulatingthe pressure of hydraulic fluid and, therefore, such adaptations shouldand are intended to be comprehended within the meaning and range ofequivalence of the appended claims.

What it claimed is:
 1. A method of controlling the pressure of ahydraulic fluid in a pilot conduit which receives fluid at an at leastsubstantially constant pressure from a constant-pressure conduit by wayof a flow restrictor and a pilot valve which is adjustable to permitoutflow of fluid from the pilot conduit through a branch conduit at avariable rate and to thus lower the static pressure of fluid in thepilot conduit in response to increasing rate of fluid outflow from thepilot conduit, comprising the steps of: establishing for the flow offluid from the pilot conduit to the pilot valve a path extending fromthe pilot conduit, first through the flow restrictor and thereuponthrough the branch conduit into and through the pilot valve; and varyingthe speed of fluid flowing through the flow restrictor to thus influencethe static pressure of fluid in the pilot conduit.
 2. The method ofclaim 1 , wherein said speed varying step includes adjusting the pilotvalve to select the rate of fluid flow from the branch conduit into thepilot valve.
 3. The method of claim 1 , further comprising the step ofcollecting the fluid flowing through the pilot valve.
 4. The method ofclaim 1 , wherein the fluid includes a lubricant.
 5. Apparatus forcontrolling the pressure of a hydraulic fluid, comprising: a pilot valvehaving an inlet, an outlet and means for varying the rate of fluid flowfrom said inlet to said outlet; a source of constant-pressure hydraulicfluid including a first conduit having an outlet; a branch conduitconnecting the outlet of said first conduit with the inlet of said pilotvalve; a pilot conduit which is arranged to receive fluid from saidbranch conduit and wherein the static pressure of fluid decreases inresponse to increasing rate of fluid outflow from the pilot conduit; anda flow restrictor establishing an evacuating path for the outflow offluid from said pilot conduit into said branch conduit.
 6. The apparatusof claim 5 , wherein said flow restrictor is installed between saidfirst conduit and said pilot conduit.
 7. The apparatus of claim 5 ,wherein said first conduit establishes a second path for the flow offluid to at least one of said first conduit and said branch conduit,said evacuating path crossing said second path.
 8. The apparatus ofclaim 5 , wherein said first conduit includes a portion constituting aportion of said pilot conduit, said flow restrictor being provided insaid portion of said first conduit.
 9. The apparatus of claim 8 ,wherein said branch conduit has an inlet positioned to receive fluidflowing along said evacuating path from said pilot conduit, through saidflow restrictor and across said first conduit, said branch conduitfurther having an outlet connected with the inlet of said pilot valve.10. The apparatus of claim 9 , wherein the inlet of said branch conduithas a first cross-sectional area and said branch conduit furtherincludes a portion adjacent said inlet of said branch conduit and havinga second cross-sectional area greater than said first cross-sectionalarea.
 11. The apparatus of claim 5 , wherein said branch conduitincludes means for opposing the development of turbulence in saidevacuating path.
 12. The apparatus of claim 5 , wherein said pilot valvehas a body an said means for varying the rate of fluid flow from theinlet to the outlet of said pilot valve comprises a valving elementwhich is reciprocable in said body.
 13. The apparatus of claim 12 ,wherein said pilot valve further comprises means for moving said valvingelement relative to said body.
 14. The apparatus of claim 13 , whereinsaid means for moving comprises at least one magnet.
 15. The apparatusof claim 14 , wherein said at least one magnet is an electromagnet. 16.The apparatus of claim 5 , wherein said first conduit and said pilotconduit include portions which are at least substantially normal to saidbranch conduit.
 17. The apparatus of claim 5 , wherein said branchconduit has at least one bend between said flow restrictor and saidinlet of said pilot valve.
 18. The apparatus of claim 5 , wherein saidbranch conduit is straight and has an inlet communicating with saidpilot conduit and an outlet communicating with the inlet of said pilotvalve.
 19. The apparatus of claim 5 , further comprising a regulatingvalve having an inlet arranged to receive fluid from said pilot conduitand an outlet, a consumer of fluid, and a control conduit connecting theoutlet of said regulating valve with said consumer.
 20. The apparatus ofclaim 19 , wherein said consumer includes a transmission in a powertrain of a motor vehicle.